This invention relates to a method and apparatus for conversion of the intensity of an optical signal into an electrical signal in digital form.
Conversion of an analog signal into a digital form is sometimes carried out by programmed algorithms in a digital computer which offer fast and accurate conversion, but require a computer. Discrete hardware converters are more often used as a peripheral device coupling an analog signal into a digital computer. Such hardware converters have taken several forms. One form, called an integrating A-to-D converter, generates a voltage ramp from a train of digital pulses integrated by an operational amplifier. When the voltage ramp reaches agreement with the analog signal, its value in the form of a binary-coded count of the pulses constitutes a conversion of the analog signal in digital form. If necessary the gain of the operational amplifier may be altered by a scale factor ranging switch.
Double voltage ramps are sometimes used in converters, requiring more circuit complexity. In the double ramp type of converter the analog input signal is integrated for a predetermined time. Then a reference signal is switched to the integrator and integrated `down` so as to discharge the voltage stored in the integrator. The time required to discharge the stored voltage is taken as a measure of the input signal's magnitude. The magnitude is expressed in digital form by counting clock pulses during the time the stored voltage is being discharged.
In some converters, the magnitude of the analog input signal is stored in a `sample-and-hold` circuit. A counter having its output terminals connected to an analog-to-digital converter is then incremented until the magnitude of the analog signal generated equals the stored signal. This type of counter works well and is inexpensive, but the maximum converting rate is well below the frequency response of the transistors used in the circuitry. For faster conversion, a register is used into which a binary 1 is stored sequentially in each binary digit (bit) position in response to clock pulses, starting with the most significant bit position. A digital-to-analog converter immediately converts the value of the binary number thus stored, and if it exceeds the input analog signal, the last binary 1 stored is changed to a binary 0 at the same time a binary 1 is stored in the next less significant bit position. This successive approximation method has the advantage of completing a conversion in a number of clock pulse periods equal to the number of bits in the register. This is of particular advantage to a computer programmer who must allow sufficient time for a conversion process.
Still other conversion methods are known and commonly used, but each is directed to the problem of converting an electric analog signal to digital form using all electronic circuits. It would be desirable to use an optical circuit to convert a signal into digital form, particularly when the signal is an optical intensity signal. Optical circuits offer the advantage of freedom from induced noise and crosstalk, voltage isolation, and also input/output branching capabilities of literally hundreds of connections. By incorporating a balancing feature in an optical analog-to-digital converter which uses the natural attenuation of light in a medium to achieve a large dynamic range, the number of sequential steps required to complete a conversion can be reduced to virtually two: comparison and verification, where the comparison is with the speed of light and verification is complete in the few nanoseconds an electronic circuit requires to close the loop and adjust a reference light source.